A battle is brewing between the Obama administration and a key group of Republican senators over U.N. Ambassador Susan Rice's response to the Sept. 11 attack on the U.S. Consulate in Benghazi, Libya.

Rice is one of the leading candidates to replace Hillary Clinton as secretary of state. However, a powerful group of senators is questioning her credibility over the incident.

For the second time in two days Rice is meeting with lawmakers, attempting to answer questions about her response to the assault.

On Tuesday, Rice conceded that she incorrectly described the attack as a spontaneous protest gone bad, when it was actually a terrorist attack.

"It seems to have been hijacked, let us say, by some individual clusters of extremists," she said in a Sept. 16 interview with ABC's "This Week."

She told lawmakers Tuesday that said she'd based those comments on the intelligence available to her at the time.

But Sens. John McCain, R- Ariz., Lindsey Graham, R-S.C., and Kelly Ayotte, R-N.H., who have questioned Rice's explanation from the beginning, said they're even more disturbed after Tuesday's meeting with Rice.

"We are significantly troubled by many of the answers," McCain said.

"The concerns I have are greater today than they were before, and we're not even close to getting basic answers," Graham said.

Acting CIA Director Michael Morell accompanied Rice to the meeting and reinforced the perception that the White House can't get its story straight about Benghazi.

Morell asserted during the meeting that the FBI modified Rice's talking points by removing a specific reference to al Qaeda.

The FBI denied the statement claiming it was actually the CIA that made the change.

Meanwhile, the White House continues to defend Rice, claiming there are no unanswered questions about Rice's comments following the Benghazi attack.

"The focus on -- some might say obsession -- on comments made on Sunday shows, seems to me and to many, to be misplaced," White House Press Secretary Jay Carney said.

President Obama said he hasn't yet decided who he'll nominate to replace Clinton as secretary of state but indicated he was considering Rice for the job.

"If I think that she would be the best person to serve America in the capacity of the secretary of state then I will nominate her," he said.

Even if Rice receives the support of every Senate Democrat she will need the support of five Republicans to gain a filibuster-proof majority.